In recent years, 16:9 wide aspect TV receivers have been widely in use to add realism of theater for audiences at home.
Recently, video softwares adapted for broadcast systems represented by wide-clear visions, VTRs, laser discs, etc. have been widely used.
EDTV-II type broadcasts are adapted for displaying on 16:9 aspect screens. While some of movies or the like recorded in laser discs and video tapes have a screen aspect wider than the 4:3 standard aspect screen as employed in the conventional TV receivers
These wide aspect programs are transmitted through a TV broadcast signal so-called letterbox image which is formed by a central image area (hereinafter referred to main image) and black belt-shape non-image areas which are disposed an the top and bottom of the main image. In such wide aspect TV receivers there are many ones which have functions of letterbox image detections, i.e., a function of detecting boundaries of the main images and the non-image areas and then blocking out the non-image areas from the display screen and thus expanding the main image both in the vertical and horizontal directions to display on the full screen of the TV receivers.
FIGS. 23a and 23b are an explanatory diagrams showing an image display for video softwares compatible with conventional wide aspect TV receivers. Wherein FIG. 23a shows an image displayed on the 4:3 aspect screen. While FIG. 23b shows the same image displayed on the 16:9 aspect screen.
In FIGS. 23a and 23b, a video software compatible with a wide aspect TV receiver provides a black band (hereinafter referred to the non-image area) on the top and bottom of the central letterbox image to maintain a compatibility with the 4:3 aspect image from the viewpoint of relationship with the current 4:3 aspect image signal as shown in FIG. 23a. Such picture is called a letterbox image. A wide aspect TV receiver has a function to detect a letterbox image and expand the picture in the vertical direction (called as a vertical expansion) so that the top and bottom non-image areas are not visible as shown in FIG. 23b when such a letterbox image is provided.
FIGS. 24a and 24b are explanatory diagrams showing a conventional image display with a caption band superposed on the lower non-image area against a video software compatible with wide aspect TV receivers. Wherein FIG. 24a shows an image displayed on the 4:3 aspect screen. While FIG. 24b shows an image displayed on the 16:9 aspect screen.
As shown in FIG. 24a, when an image with a caption band superposed on the lower non-image area is displayed on the 4:3 aspect screen, the caption band is displayed as it is, on the other hand, as shown in FIG. 24b, when an image is displayed on the 16:9 aspect screen, the vertical expansion is made likewise but in order to prevent the caption band from becoming invisible, the image is expanded to the upper non-image area and the lower portion of the caption band.
To make this operation, most wide aspect TV receivers includes a letterbox image detector for detecting a letterbox image signal.
FIG. 25 is an explanatory diagram showing features of a letterbox image.
In FIG. 25, a first feature of the letterbox image is that there are the non-image areas on the top and bottom of the letterbox image.
A second feature is that the boundary of the non-image area and the picture area (hereinafter referred to the main image) is sharp and expands across one screen image.
A third feature is that there is the main image laterally expanding around the center of the screen.
When such the first through third features are all satisfied, the letterbox image detection apparatus determines that a picture is a letterbox image.
The letterbox image detection apparatus discriminates between a picture is a letterbox image and a normal aspect image by obtaining an average luminance level and an average chrominance level across one screen image for one screen and by considering an accumulation of that value for one screen. In general, a large accumulation for several fields or frames is performed for determining whether a small accumulation across only one field or frame has stably the feature of the letterbox image, the determination is made using. In the example shown in FIG. 25, an accumulation a0 of an average luminance level and an average chrominance level satisfies all the conditions for the letterbox image and are stable over several pictures, it is determined to be a letterbox image and a microcomputer in a TV receiver is directed to perform the vertical expansion. Further, at the same time, when an accumulation of image signal of the aspect ratio 4:3 having a picture which is not a letterbox image, that is, having a picture in the non-image area (hereinafter referred to as a normal aspect image) is obtained, the letterbox image detection apparatus regards it as a normal aspect image and conveys the cancellation of the vertical expansion to the main microcomputer. If a picture cannot be determined to be a letterbox image or a normal aspect image, the picture is regarded as cannot be determined and the microcomputer is directed to maintain the current state.
FIG. 26 shows an example of a letterbox image containing a caption band.
In FIG. 26, in case of a picture which )as an edge at the lower side in the accumulation b0 of an average luminance level and an average chrominance Level for one screen and further, the luminance sharply changes in the lower non-image area, the letterbox image detection apparatus detects it and conveys that the picture is a letterbox image having a caption band to the main microcomputer. At the same time, the picture start and end position data and the caption band end position data are conveyed to the main microcomputer.
FIGS. 27a and 27b are an explanatory diagrams showing an example or a first erroneous detection of a conventional letterbox image detection apparatus.
In FIGS. 27a and 27b, a conventional letterbox image detection apparatus detects a letterbox image using an average luminance level and an average chrominance level across one screen image and therefore, when a scene changes, for instance, from a letterbox image to such a normal aspect image like the scene of the moon in a dark night as shown in FIG. 27a, the average luminance and average chrominance levels detected by the letterbox image detection apparatus are always below a threshold of a normal aspect image and the level becomes not that of a normal aspect image nor a letterbox image. As a result, the letterbox image detection apparatus outputs a determining impossible signal. Here, an ordinary vertical expansion condition when the determining is impossible is to maintain the current state and therefore, in case of the display on the 16:9 aspect screen, if an image before the scene change was fling away by a letterbox image, the display on the screen remains unchanged and an image chipping occurs in the true display as shown in FIG. 27b.
FIGS. 28a and 28b are explanatory diagrams showing an example of a second erroneous detection of a conventional letterbox image detection apparatus.
In FIGS. 28a and 28b, an example of an erroneous operation reverse to the operation in FIGS. 27a and 27b is shown. As shown in FIG. 28a, if a scene was changed from a normal aspect image (on a normal aspect display) of the scene of the moon in the dark night to a scene of the sun appearing at the center, the accumulation c0 satisfies all of the features of the letterbox image at the average luminance and chrominance levels across one screen image. In this case, the output from the letterbox image detection apparatus becomes a letterbox image and when displaying in the 16:9 aspect screen, there was such a defect that the picture was vertically expanded in spite of a normal aspect image and also an image chipping occurs.
FIG. 29 shows an arrangement of the conventional letterbox image detection apparatus.
Initially, in the interests of simplification it is assumed that a screen image having a uniform luminance like a letterbox image without caption bands is provided. This case of letterbox image is shown in FIG. 30f, which has the conventional 4:3 aspect ratio of TV receivers (here, no caption bands are included).
The luminance signal of the screen image is applied to the average value calculator 901. In response to the horizontal sync signal and the vertical sync signal, a clock pulse generator 902 generates a clear pulse (see FIG. 30g) for clearing the average value calculator 901 just before the start of every field and an average value calculation range pulse (see FIG. 30h) for showing the effective image period to the average value calculator 901. The average value calculator 901 outputs the average of the luminance signal level for each one-horizontal period (see FIG. 30c).
The output from the average value calculator 901 is applied to the input terminal of a 1H delay unit 903 and the non-inverted input terminal of a subtracter 904. On the other hand, the output from the 1H delay unit 903 is applied to the inverted input terminal of the subtractor 904. The subtractor 904 calculates a difference by a subtraction, i.e., [luminance average of current line-luminance average of 1H prior line], and supplies the calculated difference to one input of a comparator 905 (see FIG. 30b).
While the other input terminal of the comparator 905 is supplied with an upper first threshold th1 (th1 has a positive value) generated in a threshold generator 906 (see FIG. 30b). The comparator 905 provides an upper edge detection signal detected in a predetermined column to the line position data holder 907, when the difference is higher than the upper first threshold th1. The comparator 905 also provides the lower edge detection signal detected in a prescribed column to the line position data holder, when the difference is lower than the lower first threshold -th1.
A line position data generator 908 generates a line position data corresponding to the vertical position data of the screen image from the horizontal sync signal and the vertical sync signal as its input, and supplies the line position data to the line position data holder 907 (see FIG. 30a).
The line position data holder 907 holds the line position data which is found first at the field in regard to the upper edge, and holds the line position data which is found last at the field in regard to the lower edge. Accordingly, in this case, the line position data held in the line position data holder 907 has a value v1 for the upper edge and a value v2 for the lower age, as shown in FIG. 30a. These edges indicate boundaries of the non image areas and the central main image, that is the top and bottom lines or the screen image, which are operating normally.
The holding value of the line position data holder 907 is initialized to the invalid line position data value before the effective image period of the field. Since the line position data value remains the invalid line position data value when the edge in not found, the holding value is determined by the presence of the edge and the line position data holding the vertical position data. When both top and lower edge line position data holding values are valid, the screen image of the field is determined as the letterbox image.
Next, it is assumed that a letterbox image with an unchanged central main image and a caption band in a bottom non-image area, e.g., a letterbox image as shown in FIG. 30f containing a caption band. Here, the output from the average value calculator 901 will be a waveform as shown in FIG. 30d.
Since the caption band has generally a higher luminance level, the luminance level of the caption band often becomes higher than the line average luminance level of the central main image. In this case, as shown in FIG. 30e, the upper edge is normally detected as that of the main image at the level of value v1. However, the lower edge is detected as a lower edge of the caption band at the level of value v3, but not the lower edge of the main image at the level of the value v2.
According to this calculation, the caption band is determined as it contained in the central main image, so that it is impossible to operate the caption band moving function for displaying the central main image operated by using the caption band position data detector output (not shown) on a full screen and the main circuit output, and for moving the caption band to the central main image from the non image, or the caption band compression function for relatively compressing the caption band of the non image so as to display the compressed caption band on the central main image.
If an approach of holding the edge position data before the caption band is attempted, when a scene changes from a 4:3 aspect image to a letterbox image with a caption band, the picture area top and bottom line of the former letterbox image is not held so that following undesirable operations will be occurred.
After the screen image with the caption band is displayed on a full screen at the vertical position data, the screen image changes to be displayed between the true picture area top and bottom lines when the caption band disappears. Then when the caption band is appear again the screen image changes to the image display having the caption band again, so that it is hard to see because of the repeated display change in the short time.
Besides, since the edge is determined by the inter-line difference of the line luminance average, a part of the screen image where the luminance as high partially is detected as the letterbox image incorrectly (see FIG. 31), or an area having an entirely low luminance moves in the lateral direction is not detected as the letterbox image even it is moving horizontally (see FIG. 32).
Further, there is a problem that it is a letterbox image but its top and bottom lines run across several lines not parallel to the scanning line, so that it is not detected (see FIG. 33).
The conventional letterbox image detection apparatus has a problem of wrongly detecting the letterbox image, when a scene changes to an image having the luminance level of black or close to it like a scene of the moon in the dark night or when an image of high luminance level is superposed on the center of an image of the luminance level of black or close to it.
The conventional letterbox image detection apparatus has also a problem that it can not detect the true top and bottom picture area end data when there is the caption band, or commits an error in discrimination of letterbox image or other image for the images as shown in FIGS. 31 through 33.